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Disconnect wiring harness at AC compressor. Turn truck on and turn AC on. Use a multimeter or test light and connect one lead to ground. Probe both pins of the wiring harness. One of them should indicate 12V on the multimeter, or make the test light light up. That's the wire you want.
The Dark Green/Orange wire is hot whenever the A/C is selected and the compressor is engaged. If you want the fan(s) to run continuously when A/C is selected use the Pink wire on the input side of the low pressure/clutch cycling switch.